1. Field of the Invention
The present invention generally relates to probe style radiometers, and more particularly to a high dynamic range, cost effective and safe probe radiometer for measuring ultraviolet (UV) radiation in a production environment.
2. Background Description
Use of ultraviolet light in the printing and coatings industry has grown rapidly over the past several years. Special ink and coating formulations will polymerize, or xe2x80x9ccure,xe2x80x9d with exposure to light in the ultraviolet (UV) region of the spectrum. Measurement of the amount and intensity of UV exposure a product receives during the manufacturing process is paramount in establishing and maintaining process control. UV radiometers are used to measure the amount of UV light to which a product is exposed. There is much variation in the configuration of UV curing production lines. As a result, many different types of radiometers are used. The different configurations are driven by both physical considerations and by levels of UV irradiance.
One type of radiometer, which is often used to obtain UV readings in hard-to-reach locations, is the xe2x80x9cprobexe2x80x9d style of radiometer. The xe2x80x9cprobexe2x80x9d style radiometer typically consists of a long tube, which houses the light-capturing optics at the tip, and is connected to a body containing circuitry which measures the amount of UV light entering the tip of the instrument.
Most probe style radiomenters have their light sensing probes constructed using a quartz rod enclosed in a round stainless steel tube. This design has several problems. The first problem is cost and performance. The quartz rod must be constructed of high purity fused silica in order to provide high transmission to all wavelengths in the UV region. This drives the cost of the quartz rod upward, as the better the transmission the higher the cost. Moreover, a quartz rod may break if struck with a sharp blow. Other probe designs which use quartz fibers are not as susceptible to sharp blows but instead are very susceptible to changes in light output associated with bending of the fibers. Also, the fibers tend to be quite expensive when made of fused silica.
A second problem is related to the metallic surface of the sheath or cladding which encases the quartz rod or fibers. This metallic cladding presents serious electrical shock and arcing hazards when the probe is inserted into an operating UV environment. UV sources utilize high power inductors and capacitors, and high starting voltages and gas plasmas with up to 10 KW power potential. Contact by the operator with these sources is very hazardous. Further, shorting these power sources through a metal conductor to a machine body or other ground potential presents a hazardous arcing potential.
A third problem relates to the cross-sectional shape of the probe. Beacuse of their use of quartz rods and fibers, most probe designs have a round cross-section. This is undesirable since because rotation of the tip of the probe around its longitudinal axis will cause changes in the measured reading. The round cross-section thus makes it difficult for a user to precisely position the probe for properand consistent measurement.
It is an object of the present invention to provide an improved probe-style radiometer for detecting ultraviolet radiation, and more specifically one which is more economical to make and safer to use and demonstrates improved performance compared with conventional probe-style radiometers.
It is another object of the present invention to provide a probe-style radiometer having a probe rod which is made from an economical material that is resistant to breakage and which at the same time reliably captures and transmits all wavelengths in the ultraviolet region for detection.
It is another object of the present invention to provide a probe-style radiometer which is constructed to prevent a user from experiencing an electrical shock.
It is another object of the present invention to provide a probe-style radiometer which uses a probe rod having a cross-section which can be more precisely positioned by a user during UV detection, thereby enabling the invention to obtain a more accurate and consistent radiation reading compared with conventional probes with a round cross-section.
These and other objects of the invention are achieved by providing a radiometer which has a probe having a generally rectangular cross-section attached to a body which serves both as a handle and a housing for holding the probe electronics. A top surface and a bottom surface of the body preferably have larger width dimensions than the side or edge surfaces, and the overall dimensions of the body are selected to provide a comfortable fit in a user""s hand.
The radiometer also includes a liquid crystal display (LCD) formed along one surface of the body, and control switches preferably in the form of two membrane pushbuttons along another one of the body surfaces. The pushbuttons may be located for convenient operation by the user""s thumb or index finger. The body can be held either with an xe2x80x9coverxe2x80x9d or xe2x80x9cunderxe2x80x9d hand type grip, so that the membrane switches are easy to operate with either hand. Information indicative of the UV irradiance detected by the probe is displayed on the LCD, as well as other information including modes of operation, units of measure, and other probe functions. The instrument is preferably battery powered and so that it does not require external wiring to a power source.
In use, the operator grasps the body of the probe in one hand and places the end of the measurement probe in the area in which the UV radiance is to be measured, which, for example, may be a UV curing chamber. Light enters the probe through a small aperture in the tip of the probe. The aperture admits all wavelengths into the interior of the probe through a ground quartz or glass diffuser window which seals the probe interior from outside solids, liquids and gases which may contaminate the interior of the probe and interfere with the UV radiation. The ground quartz or glass window also provides diffusion of the incoming rays so that the angular response of the probe is nearly cosine in nature. Light entering the probe window strikes a mirror inclined (e.g., at a 45xc2x0 angle) to the quartz window, so that light striking the top surface of the probe is reflected down the length of the probe.
Circuitry within the body measures and displays, in numerical form, the UV irradiance collected at the tip. Irradiance, total energy, and time is measured and displayed on the LCD on the body. At the end of the probe, a UV filter eliminates all wavelengths except those of interest and allows the UV wavelengths to strike a silicon photodiode or other photodector. The photodetector converts the UV radiation into electrical current proportional to the UV radiation striking the detector. The current from the detector is preferably received by a very wide dynamic range (22-bit) analog-to-digital (A/D) converter which converts the current to numerical form.
The invention includes several other features which are not used in conventional probe style radiometers. These include:
Very Wide Dynamic Range and Automatic Operation
The radiometer of the present invention has enhanced sensitivity which enables it to detect UV light in a range from several microwatts to ten watts. This wide dynamic range is accomplished automatically with no requirement by the user to switch gain settings of any kind. This is made possible by the integration of 400, filtered, 20-bit A/D samples/coupled with a software-controlled, 60:1 hardware gain range. Those skilled in the art can appreciate that other arrangements may also be used.
In accordance with microcontroller software, the display will automatically adjust itself with a floating decimal point and proper units to present information in either milliwatts or watts. Preferably, the LCD display display values as low as 0.001 mW/cm2. If desired, the control software of the radiometer may automatically change UV detection ranges as the irradiance level in the measuring area is detected as changing. This auto-ranging may be achieved by the control software automatically adjusting the displayed data to provide the maximum displayed resolution without overloading the displayed range.
Implemented in the signal detection circuitry is a combination high resolution A/D converter and integrating amplifier. The amplifier portion of the A/D converter has a gain switching capability which is controlled by an on-board microcontroller. The microcontroller uses a signal processing algorithm to select the proper gain range, calibration method and filtering process to provide the user with a high dynamic gain range instrument which does not require user intervention.
Rigid Probe Design
The high cost of quartz rods or fibers used in conventional probe-style radiometers is avoided in the present invention by the use of a simple, inexpensive turning mirror placed at the light-sensing end of the probe. Light enters the probe tip where it strikes a mirror placed, for example, at a 45xc2x0 angle and is reflected down the long axis of the probe (e.g., the light guide) to the body. From there, it passes through one or more filters and strikes a photodetector where it is converted to an electrical signal, and then that signal is processed to achieve a final UV radiation reading by the probe electronics.
In addition to cost savings, the design of the present invention is easier to assemble and more robust than a quartz rod which may break if struck a sharp blow. The preferably rectangular or square cross-section of the probe rod is also advantageous. The shape of the square probe, for example, allows for better control of rotational motion of the probe tip when in use; that is, when the probe is inserted into a square opening of an area containing UV radiation. Since the effective sensing area of the probe tip is approximately cosine in nature, rotating the tip while measuring a source will change the reading. It is therefore important that the tip not rotate while in use. The square shape when inserted in a slightly larger square hole assists the user in this task.
Electrically Isolated Probe
The probe design of the present invention avoids electrically-related hazards. Although the probe preferably has a metallic core, it is clad in a very hard, durable, electrically non-conductive coating which is able to withstand, for example, up to 5 KV and can operate in temperatures of up to 1200xc2x0 F. This coating may, for example, be a ceramic-based material. The probe itself is also electrically isolated from the portion of the instrument in physical contact with the operator. The isolation is obtained by using an electrically non-conductive block that mechanically connects the probe to the instrument body.